1. Field of the Invention
The present invention relates to a WDM ring network in which a center node and a plurality of remote nodes all connected together in ring form communicate with one another using a WDM technique.
2. Description of the Related Art
Conventional WDM ring networks include a type A1 in which each remote node has a light source and a type A2 in which a center node has a multi-wavelength generator arranged therein, whereas each remote node receives, modulates, and transmits continuous-wave (CW) light transmitted by the center node. In connection with an arrangement for allowing each remote node to split and insert (add and drop) an optical signal, the conventional WDM ring networks also include a type B1 in which an optical filter is used to demultiplex/multiplex an optical signal of a predetermined wavelength transmitted through an optical fiber transmission path, and a type B2 in which an optical coupler is used to split a part of optical signal power and then selectively receive an optical signal of the predetermined wavelength. For example, the WDM network described in Japanese Patent Application Laid-open No. 7-231305 (1995) is known as a combination of the types A2 and B1.
This publication discloses a multi-wavelength generator that modulates a vertical mode from a multimode laser or output light from a single mode laser to cut out its sidebands. However, with such a light source, the number of vertical mode frequencies or sideband frequencies having optical power sufficient for communication is limited. Further it is difficult to obtain flat power with these frequencies. Accordingly, such a light source cannot be used for a multi-wavelength generator for a network using more than 100 wavelengths.
To solve this problem, a multi-wavelength generator has been provided which generates multi-wavelength light at once by using an electric signal (for example, a sine wave) having a particular cycle period to modulate the phase and intensity of light having a single central wavelength to thereby generate sidebands. Such a technique is disclosed in (1) M. Fujiwara, et al., “Flattened optical multicarrier generation of 12.5 GHz spaced 256 channels based on sinusoidal amplitude and phase hybrid modulation”, IEE Electronics Letters, Vol. 37, No. 15, pp. 967–968, July 2001.
According to Document (1), a multi-wavelength generator is composed of an optical generating section and a multi-wavelength modulating section. The optical generating section has a semiconductor laser (LD) that generates light of a single central wavelength. The multi-wavelength modulating section is composed of an intensity modulator that modulates the intensity of output light from the optical generating section, a phase modulator that modulates the phase of the output light (these modulators are arranged in an arbitrary order), a period signal generator that generates a predetermined period signal (sine wave) applied to each of the modulators, and a voltage adjusting section that adjusts an applied voltage and bias voltage in the period signal.
The optical generating section may comprises n semiconductor lasers (LD) that generate lights of optical frequencies f1 to fn spaced at even intervals, and a multiplexer that multiplexes the laser lights into output light. In this case, the multi-wavelength modulating section generates sidebands around each central wavelength and can further generate multi-wavelength light over a wideband at once. This configuration enables the number of sidebands to be increased by installing an additional light source at an input port of the multiplexer. Thus, this configuration allows additional components to be easily installed.
Further, a linear optical repeater that does not insert or split wavelengths is provided with a gain control function of keeping the output power of signal light at a fixed value in order to maintain desired transmission quality even with a variation in optical fiber loss or number of wavelengths within a repeating section. The linear optical relay is basically composed of an optical amplifier having its gain controlled to a fixed value, a variable optical attenuator, and feedback control means therefor.
The feedback control means monitors the total optical power and the number of wavelengths after light has passed through the variable optical attenuator to adjust attenuation provided by the variable optical attenuator so that the total optical power divided by the number of wavelengths is equal to desired signal optical power per channel. This enables the signal optical power per channel to be kept at a fixed value in spite of a variation in loss or number of channels.
A problem with the multi-wavelength generator is the superimposition of multiple sidebands generated around each of the central wavelengths of a plurality of light sources. To avoid this problem, large intervals must be used to space the optical frequencies of the light sources. Alternatively, it is necessary to remove unwanted sidebands generated around the central wavelengths of the light sources.
However, the former method may reduce the efficiency with which the optical frequencies are used. The latter method may complicate the configuration of the multi-wavelength generator and may degrade an optical SN ratio as a result of an optical loss if an optical filter or the like is used.
On the other hand, in a remote node that uses an optical coupler or the like to insert and split wavelengths, the gain control function requires a mechanism that adjusts the power of transmitted signal light but also the power of signal light inserted into the node, to desired levels. Further, in a remote node with such a function of inserting and splitting wavelengths, the number of wavelengths varies between the input and output thereof. Accordingly, the above conventional technique still has room for improvement; it must be clarified where in the network wavelength number information should be used, what control procedure should be taken, and what should be controlled.